A typical spark plug comprises an insulator body enhousing a center electrode and a metal shell crimped about the insulator body. The shell is threaded for mounting the spark plug in a bore of a cylinder head of an internal combustion engine and also comprises a side electrode. The side electrode depends from the shell off-center from the center electrode. Because of this, it is necessary to size the bore not only to receive the center electrode but also to accommodate the side electrode. At the same time, there has been a trend toward downsizing the engine and to adding features, such as multiple valves and larger gas passages, that reduce the space available on the face of the cylinder head for the spark plug. Thus, there is a need for a spark plug that reduces the diameter of the spark plug bore in the cylinder head.
It is an object of this invention to provide a spark plug assembly for an internal combustion engine that includes a ground electrode that is permanently affixed to the cylinder head and a center electrode element replaceably mounted in the cylinder head and cooperating with the ground electrode to provide the necessary gap for generating a spark for engine operation. By eliminating the necessity to accommodate the ground electrode through the cylinder head, this invention allows the diameter of the cylinder head bore to be sized to a minimum necessary to accommodate the center electrode element.